1. Field of the Invention
The present invention relates to a semiconductor device and a control method thereof, and particularly to a control method at the time of activating a semiconductor device.
2. Description of Related Art
A demand for a semiconductor memory device such as a DRAM (Dynamic Random Access memory) to increase operation speed and degree of integration and to reduce power consumption has been met mainly with a development of the micro-fabrication technology. However, as the operation voltage decreases with downscaling of circuits, a fluctuation of threshold voltage and an off-state leak current (a consumption current at standby) due to the short channel effect become more problematic. Therefore, a process for measuring the threshold voltage and the like of a transistor and optimizing internal voltages for each semiconductor memory device has become essential.
To set optimum internal voltages for each semiconductor memory device, it is necessary to store optimum values in the semiconductor memory device. A fuse element is often used as a memory element for storing the values. The fuse element is electrically in a conductive state in an initial state, and is changed to a nonconductive state (an insulation state) with line-breaking by an irradiation with a laser beam. Information of one bit can be represented by the conductive state and the nonconductive state of one fuse element.
In recent years, an antifuse element is used instead of the fuse element in many cases. The antifuse element is in a nonconductive state in an initial state, and is changed to a conductive state with an insulation-breakdown by an application of a high voltage. Japanese Patent Application Laid-open No. 2002-76855 discloses a technology for adjusting internal voltages by using an antifuse element.
Because the antifuse element is smaller than the fuse element in size and there is no damage on a passivation layer due to programming, it is expected to be in widespread use in the future.
However, even after the insulation breakdown, a resistance of the antifuse element is relatively large. The resistance of the antifuse element in the “conductive state” is hundreds of times as large as the resistance of the fuse element in the “conductive state”. Therefore, the antifuse element requires more time to read out data than the fuse element. In addition, a circuit for reading out data from the antifuse element, if not supplied with a sufficient internal voltage, cannot read out data in a stable manner. Because the voltage is not stabilized right after activation, there is a problem that it is hard to read out data in a stable manner.